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Immunology · Innate Immunity

Oxygen-independent bactericidal factors

Oxygen-independent bactericidal factors are a group of pre-existing intracellular mechanisms and molecules that ensure the destruction of engulfed microorganisms without the involvement of oxygen derivatives. They are stored in pre-formed phagocyte lysosomes and released into the phagolysosome, disrupting the pathogen's structure or blocking its metabolism.

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Acidification of the environmentV-ATPase activity sharply decreases the pH inside the phagolysosome to 4.5–5.0.
OriginUnlike ROS, these factors are not synthesized de novo, but are derived from pre-formed lysosomes.
DegranulationThey are capable of acting extracellularly if released into the intercellular space during degranulation.
3 min readFor medical studentsUpdated Sep 25, 2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesReviewed by medical editor: Aulov A. A., general practitioner · 09/24/2026

Classification of oxygen-independent factors

Phagocytes possess a vast array of tools that do not require an oxidative burst for their activation. The main oxygen- and nitric oxide-independent factors include:

  1. Factors of local acidification. A key event is the decrease in pH to 4.5–5.0. This process is driven by a specialized enzyme, V-ATPase. The acidic environment is inherently detrimental to many bacteria while simultaneously creating optimal conditions for the activity of intracellular enzymes.
  2. Cationic proteins. A large group of molecules that disrupt bacterial cell walls and membranes. This includes lysozyme, lactoferrin, BPI proteins, as well as the serprocidin family (cathepsin G, elastase, azurocidin, and proteinase-3).
  3. Acid hydrolases. Phagocyte digestive enzymes. This group includes myeloperoxidase, 5'-nucleotidase, β-arylsulfatase, β-glucuronidase, and acid glycerophosphatase.
  4. Bactericidal peptides. Short protein molecules with direct antimicrobial activity. The most important representatives are α- and β-defensins, as well as cathelicidins.
  5. Competitive metabolic inhibitors. Substances that deprive bacteria of nutrients essential for their reproduction and survival.

Localization and mechanism of action

The primary site where bactericidal factors exert their action is the phagolysosome, a compartment formed by the fusion of a phagosome (a vesicle containing the engulfed microbe) and a lysosome.

  • Intracellular origin. Enzymes and bactericidal peptides are stored within the cell in advance. They enter the phagolysosome from a pool of pre-formed lysosomes.
  • Extracellular action. An important feature of all bactericidal factors is their ability to act outside the phagocyte. This occurs as a result of degranulation—the release of granule contents to the exterior.

Note: the action of these factors must be strictly distinguished from extracellular cytolysis mechanisms, which are not inherently linked to the phagocytosis process.

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Comparison with oxygen-dependent mechanisms

Although oxygen-independent molecules are extremely vital, oxygen-dependent factors play the most potent role in microbial killing.

Unlike pre-formed lysosomal enzymes, reactive oxygen and nitrogen species are generated strictly de novo. The main event here is the oxidative burst, which represents a rapid (taking fractions of a second) and highly productive chain of chemical reactions.

The process is triggered by the enzyme NADPH oxidase (synonyms: phagocyte oxidase or Phox), which catalyzes the formation of highly reactive free radicals and ions from oxygen. This enzyme assembles directly on the phagolysosomal membrane under the influence of intracellular signals from Fc receptors.

As a result of the oxidative burst, the following are formed:

  • Reactive oxygen species (ROS): superoxide anion (O2-), hydrogen peroxide (H2O2), hydroxyl radical (OH), hydroxide ion (OH-), hypochlorite ion (OCl-), singlet oxygen ('O2), and ozone (O3).
  • Reactive nitrogen species (RNS): nitric oxide (NO), peroxynitrite (OO*NOI), and others. Moreover, NO and its metabolites can be formed and exert their bactericidal effect even outside the cellular granular apparatus.

Briefly: answers to topical questions

What is the mechanism of the antimicrobial action of defensins on a bacterial cell?

  • Defensins exhibit antibacterial activity and possess amphipathicity due to the spatial separation of basic and hydrophobic amino acids.

Class and exam questions

Why is acidification of the phagolysosomal environment necessary?

Lowering the pH to 4.5–5.0 via the V-ATPase enzyme is necessary to create a bactericidal environment and activate the phagocyte's acid hydrolases.

Can intracellular factors kill bacteria outside the cell?

Yes, all the listed bactericidal factors are capable of acting in the extracellular space if the cell undergoes degranulation.

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What else to study on this topic

  • Mechanism of activation and assembly of NADPH oxidase (Phox)
  • Stages and biochemistry of the oxidative burst
  • Properties and types of reactive oxygen species (ROS)
  • The role of Fc receptors in intracellular signal transduction
  • Synthesis and extragranular action of reactive nitrogen species

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